Simultaneous Generation of Methyl Esters and CO in Lignin Transformation

Abstract Lignin is an abundant renewable carbon source. Due to its complex structure, utilization of lignin is very challenging. Herein, we describe an efficient strategy for the simultaneous utilization of lignin, in which the methoxy groups in lignin react with carboxylic acids to generate methyl carboxylates and the other alkyl and phenyl carbons react with oxygen to predominantly form CO that can be used directly in carbonylation reactions. The method was applied to the methylation of various functionalized aryl and alkyl carboxylic acids, including natural compounds, to produce valuable chemicals, including pharmaceuticals. No solid or liquid residues remain after the reaction. Mechanistic studies demonstrate that a well‐ordered C−C and C−O bond activation sequence takes place to realize total transformation of lignin. This work opens a way for transformation of the entire lignin polymer into valuable products, exemplified by the synthesis of the pharmaceutical, Ramipril, on a gram scale.

anhydrous Mg 2 SO 4 , and finally concentrated in vacuo to give alcohols C. Further purification was conducted using silica gel column chromatography if necessary.
(1) 2-Methoxyphenol (30 mmol), ethyl bromoacetate (30 mmol) and K 2 CO 3 (60 mmol) were added into acetone (50 mL). The mixture was refluxed for 14 h until the substrates were fully consumed. After cooling to room temperature, the mixture was filtered and concentrated in vacuo to yield the solid product ketone B (ethyl 2-(2-methoxyphenoxy)acetate). (2) Veratraldehyde (10 mmol) and B (11 mmol) were dissolved in toluene (30 mL) and dried by azeotropic distillation thrice. Then the solid mixture was dissolved in dry THF (12 mL) and cooled to -78 °C in a dry ice/actone bath. Freshly prepared lithium diisopropylamide (LDA) solution (see below) was added dropwise into the mixture at -78 °C, and the reaction mixture was stirred at -78 °C for 2 h. After warming to 0 °C, the reaction was quenched with 100 mL saturated NH 4 Cl aqueous solution. The aqueous phase was extracted with ethyl acetate (3×100 mL). The organic phase was washed with brine, dried with anhydrous Mg 2 SO 4 and concentrated in vacuo. The crude product was purified using silica gel column chromatography, generating colorless oil C. (3) Product D was generated from the reduction of C with NaBH 4 as described above. 30 mol% NaBH 4 was used as reductant to reduce C into D in methanol solution at 0 °C. Further purification was conducted using silica gel column chromatography to obtain pure 1,3-diol lignin model compound D.
Fresh LDA solution was prepared by using the following procedure: when a solution of diisopropylamine (11 mmol) in dry THF (18 mL) was cooled to -78 °C, n-butyllithium solution (2.0 M in cyclohexane, 5.5 mL) was added at -78 °C. The LDA solution was used after warming to 0 °C.
Acetylation of phenols [4] : pyridine was used as base to promote the acetylation of phenol. Phenol (100 mmol), acetic anhydride (200 mmol, 2.0 equiv.) and pyridine (60 mmol, 0.6 equiv.) were stirred for 2 h at 25 °C. The purification process was the same as that used for the formylation of phenols.

Synthesis of isotope labeled phenols
Preparation of guaiacol with deuterated methoxy group [5] : (1) catechol A (20 mmol), benzyl chloride (20 mmol), anhydrous K 2 CO 3 (30 mmol), and a catalytic amount of KI (1 mmol) were added to anhydrous acetone (100 mL) and refluxed for 10 h. After reaction, the reaction mixture was filtered, washed with brine, dried with anhydrous Mg 2 SO 4 , and concentrated in vacuo to give partially substituted phenol B. (2) Phenol B (10 mmol), CD 3 I (30 mmol), and K 2 CO 3 (30 mmol) were added to anhydrous acetone (100 mL) and refluxed for 10 h. After reaction, the mixture was filtered and concentrated in vacuo to give ether C.
(3) Iodotrimethylsilane was used as a reductant to remove the benzyl protecting group. Ether C (10 mmol) was dissolved in CH 3 CN (50 mL), heated to 50 °C under dry N 2 . Iodotrimethylsilane (40 mmol) was added to the solution. The reaction was stirred at 50 °C for 20 min and quenched with methanol (10 mL). The reaction mixture was transfered to a two-phase system containing ethyl acetate (100 mL) and H 2 O (100 mL). The aqueous phase was extracted with ethyl acetate (3×100 mL). The organic phase was washed with brine, dried with anhydrous Mg 2 SO 4 , and concentrated in vacuo to afford guaiacol D.
Preparation of guaiacol 18 O labeled methoxy group [5][6] : (1) the process to protect the hydroxyl group with benzyl chloride is the same as step 1 described above. (2) Ether B (3 mmol), CuI (0.3 mmol), 1,10phenanthroline (phen, 0.9 mmol) and KOH (10 mmol) were added to DMSO/H 2 O (1:1, 3 mL) under N 2 , and reacted at 100 °C for 24 h in a sealed tube. After reaction, the mixture was transfered to two-phase system containing ethyl acetate (20 mL) and H 2 O (20 mL). The aqueous phase was extracted with ethyl acetate (3×20 mL). The organic phase was washed with brine, dried with anhydrous Mg 2 SO 4 , and finally concentrated in vacuo to give crude product. Further purification was conducted using silica gel column chromatography to afford pure phenol C. (3) Methylation of phenol C and (4) deprotection of D was the same as steps 2 and 3 described above.
Preparation of guaiacol 18 O labeled hydroxyl group [6] : hydroxylation of 2-Iodoanisole to produce the labeled guaiacol is the same as step 2 above.

Extraction of organosolv lignin
Organic lignin was extracted according to previous reports. [2] Specifically, wood powders (<0.5 mm) were dried in vacuo at 80 °C for 24 h. The dried wood powder (200 g), 1,4-dioxane (1.5 L), and 2 M HCl (160 mL) were added to a three-necked flask. The reaction mixture was maintained under N 2 and heated to 110 °C and held for 60 min. After cooling to room temperature, the mixture was filtered and washed with 1,4-dioxane. The filtrate was concentrated into a sticky liquid (ca. 200 mL), which was subsequently mixed with acetone/water solution (9:1, ~250 mL). The resulting mixture was slowly added into rapidly stirred water (2.5 L) to genarate crude lignin powder. The resulting precipitate was flitered and redissolved in minimal acetone/methanol (9:1) solution and precipitated by slowly pouring it into rapidly stirred diethyl ether (2 L). The lignin precipitate was dried in vacuo at room temperature.

Characterization methods
The qualitative analysis of liquid products was conducted using GC-MS (Agilent 5975C-7890A, equipped with electron impact ionization mass spectrometer) and by comparison with authentic samples. The conversion of carboxylic acids and GC yields of corresponding ester products were quantitatively analyzed using GC (Agilent 7820, equipped with a hydrogen flame-ionization detector, full electric pneumatic control, inlet temperature 280 °C) based on internal standard curves and areas of integrated peak area. Quantitative analysis of gaseous products was conducted using GC (Agilent 7820, equipped with thermal conductivity detector) by comparison with authentic gas samples.
NMR spectra were recorded on Bruker Avance 400, 500 or 600 spectrometers equipped with 5 mm pulsed-field-gradient (PFG) probes. Pulse program of the Heteronuclear Single Quantum Coherence (HSQC) experiments was hsqcetgp. The spectral width of 13 C is from -10−210 ppm. The resonance band of tetramethylsilane (TMS) or solvents (DMSO or CD 3 Cl) was used as the internal standard. Spectra were recorded at 298 K. For preparation of samples for HSQC experiments, DMSO-d 6 was used as solvent. After reaction, stoichiometric sodium hyposulfide (0.6 mmol) was added to consume the residual iodine. Then the mixture was flitered through silica gel to remove any undisolved Cu catalyst, base, and ligand. The filtrate was used directly for HSQC experiments. For quantitative 13 C NMR spectroscopy, chromium(III) acetylacetonate (Cr(acac) 3 , ~2 mg) was added to the lignin solution (~100 mg, 0.5 mL DMSO-d 6 ) to completely relax the nuclei.

Quantitative methods
The molar concentration of the methoxy groups and other phenyl and alkyl carbons in lignin is determatined by quantitative 13 C NMR spectroscpy using trioxane as internal standard (Figs. S4-6), calculated with the specified weights and integration of the 13 C NMR spectra. In a typically reaction, lignin containing 1.5 equiv. of methoxy groups was used as the methylation reagent. For example, for a 0.2 mmol scale reaction, 45 mg beech lignin (methoxy group, 0.3 mmol; other carbons, 1 mmol) was used.
Yield (%) of liquid ester products is based on the carboxylic acid substrate: n ester was determatined by GC based on internal standard curves and areas of integrated peak areas of internal standard and esters, or from the weight of the isolated products (specified high boiling point products).
Yield (%) of CO gas product was caculated using: n CO was determatined by GC using an external standard method obtained from the measurement of several standard CO sample with different concentrations.
: mole of other carbons in lignin used. For typical reaction, lignin (45 mg) was used, which contains other alkyl and phenyl carbons (1.0 mmol). Table S1. Carbon balance of reaction process.
Nearly all the Cu salts catalyze the methylation of p-phenylbenzoic acid. However, CuO nano powder (aerodynamic particle sizer, APS, 30-50 nm, purchased from Alfa Aesar) was the most efficient catalyst. Table S3. Optimization of reaction conditions by screening ligands.
Several modified phenanthroline ligands were tested. Methoxy group substituted phenanthroline (Ophen, L4) showed higher activity compared to the others (L1-3, L5-6). Bulky bathophenanthroline (L7) and other bidentate nitrogen ligands (L8-L12) showed poor catalytic activity.  Overall, inorganic bases are more efficient than organic bases. K 2 CO 3 , and K 3 PO 4 have similar efficiency and K 2 CO 3 was chosen as it also affords a higher yield of CO, has a lower hygroscopicity and is the least expensive.  Iodine containing additives have better activity than bromine containing additives. Their influence on the reaction is discussed in the mechanism part. Note that water absorbing agent molecular sieve (4A) decreases the yield of products.    No signals of Cu/ligand complex were detected by ESI-MS, which further confirms the insolubility of CuO in our reaction system. Although we cannot exclude the possibility that homogenous species are formed under the reaction conditions, ligands are known to improve the activity and/or selectivity of heterogeneous catalysis as they modify the sterics and electronics at the inorganic-organic interface. For regeneration of L4, H 2 O (50 mL) and CH 2 Cl 2 (50 mL) was added to the reaction mixture after reaction. Sodium hyposulfide (3 mmol) was added to consume residual iodine. The CuO catalyst was removed by centrifugation. Then, the aqueous phase was extracted with CH 2 Cl 2 (2 × 50 mL). The combined organic phase was dried with Na 2 SO 4 , concentrated under vacuum to give dark brown solid organics. The solid organics was washed with (3 × 5 mL) and ethyl acetate (3 × 5 mL) to remove the organic products. Finally, the solid was purified by silica gel flash chromatography (2% MeOH in CH 2 Cl 2 as eluent) to afford L4. Figure S4. Quantitative 13 C NMR spectrum of pine wood lignin. Trioxane was used as an internal standard to calculate the content of the methoxyl groups. Pine lignin (103.6 mg), Cr(acac) 3 (2.3 mg), and trioxane (5.0 mg) in DMSO-d 6 (0.5 mL). The content of methoxyl group is 3.75 mmol/g lignin. The content of other carbon atoms is 23.26 mmol/g lignin.    When lignin was added into the standard reaction, CO is detected confirming that it is derived from lignin decomposition.   [8] (2) Pinacol phenylboronate (0.2 mmol), PdCl 2 (PPh 3 ) 2 (0.01 mmol), triethylamine (Et 3 N, 0.04 mmol), butanol (2 mL), gas mixture (1 atm), 40 °C, 24 h, GC yield. [9] (3) 1-phenylethane-2,3-diol (0.2 mmol), PdCl 2 (0.02 mmol), CuCl 2 (0.4 mmol), NaOAc (0.04 mmol), dichloromethane (DME, 5 mL), gas mixture (1 atm), RT, 48 h, GC yield. [10] (4) Boronic acid (0.2 mmol), PdCl 2 (PPh 3 ) 2 (0.005 mmol), CuCl (0.01 mmol), DMF (2 mL), gas mixture (1 atm), 100 °C, 24 h, GC yield. [11] The gas mixture could be directly used in carbonylation reactions without purification. Four carbonylation reactions were performed, phenylacetylene (d), pinacol phenylboronate (f), 1-phenylethane-2,3-diol (h), and boronic acid (j) were tested and successfully generated the corresponding carbonylation products, i.e. alkynoate (e), benzoate (g), cyclic carbonates (i), and biphenyl ketones (k) in high yield (71%-99%).     Pathway a (Fig. S15a): dimethyl carbonate is a well-known methylation agent. [12] Deprotonated acid under alkaline conditions can break the CH 3 −O bond by nucleophilic attack to the methyl group and transfer to the methyl ester. Pathway b (Fig. S15b): as for methyl formate, Cu catalyzed rearrangement reaction under alkaline conditions can cause its decomposition to methanol. [13] The conversion of methanol and carboxylic acids to produce methyl ester will happen according to a lierature report. [14] Pathway c (Fig.  S15c): for relatively stable methyl benzoate, the CH 3 −O bond of methyl benzoate is not easily cleaved by decomposition, but can be activated via oxidative addition with a metal catalyst in a weak equilibrium. [15] The generated catalytic intermediate is highly active and can react with other intermediates that promote the following reaction. Figure S16. Isotope labeling experiments were carried out using 18 O labeled guaiacol, or H 2 18 O. Abundance (%) of isotope is listed. 18 O labeled oxygen was highlighted in red.
Oxygen exchange between H 2 O and the substrates or intermediates was not observed and addition of H 2 18 O did not generate 18 O labeled ester products (Fig. S16).   The crude reaction mixture (200 μL) was mixed with 5,5dimethyl-1-pyrroline N-oxide (DMPO, 3 M in DMSO) and an EPR spectrum was recorded. EPR signal (g 0 = 2.004) of DMPO-• OOH was identified, which demonstrates the formation of superoxide radical during the reaction of O 2 and Cu catalyst. [17] The activation of C=C bonds is required for the decomposition of benzoquinone. It has been reported that heterolysis of I 2 affords I + and I -(or I 3 -). [18] I + reacts with the C=C bond to form a three-membered iodonium ion containing ring 2. [19] This process activates C=C bond to afford the carbon-centered cation 3. [19a] Superoxide radical generated by the oxidation of Cu I catalyst react with 3 to give superoxide radical intermediate 4. [20] After reduction of 4 to peroxide anion 5, intramolecular nucleophilic attack form dioxetane intermediate 6. Fragmentation of dioxetane 6 lead to the C−C bond cleavage to give 7 and 8. [21] After hydrolysis or oxidation, carboxylic acid products 9 are obtained. [22] Figure S23. Proposed reaction pathway from phenol to methyl ester and CO products.
The reaction pathway for the depolymerization of lignin to phenols was described in Fig. S11. This figure shows the mechanism from the oxidative fragmentation of phenols. The phenyl carbon and methoxy carbon) atoms were both labelled, and both could be upgraded to CO and methyl ester, respectively. Methoxy substituted phenol 1 decomposes to methyl formate 5 and CO mediated by benzoquinone 2, [23] followed by fragmentation to polycarbonyl compound 4 catalyzed by iodine catalyst. [14, 19a] Thermal decomposition of 4 gives methyl formate 5 and CO. The following step is similar as the Chan−Lam type cross coupling reaction for alkylation of heteroatom nucleophiles such as carboxylic acids and amines catalyzed by Cu Catalysts. [24] Organic Cu III species 8 may be formed by the oxidative addition of 5 to the Cu I catalyst 9. Carboxylic acid is activated by the Cu II catalyst 6. Transmetallation between the Cu III intermediate 8 and carboxylate Cu II 7 generates intermediate 10, followed by reductive elimination to produce methyl ester products. Iodine promotes the redox reaction between the Cu catalyst and O 2 and enables recycling of Cu catalyst during the reaction. [14,25] NMR spectra

Isotope labeling compounds:
In DMSO-d 6 In DMSO-d 6 In DMSO-d 6 In DMSO-d 6

Lignin model compounds:
In